The invention relates to a transmission system comprising a transmitter which includes an encoder for deriving an encoded signal from a quasi-periodic signal, transmitting means for transmitting the encoded signal to a receiver, which receiver includes a decoder for deriving a reconstructed signal from the encoded signal.
The invention additionally relates to a transmitter, a receiver, an encoder, a decoder and a codec to be used in such a transmission system. A transmission system as defined in the opening paragraph is known from the journal article "Methods for Waveform Interpolation in Speech Coding" by W. B. Kleijn in Digital Signal Processing, vol. 1, no. 4, October 1991, pages 215-230.
Transmission systems of this type are used, for example, for transferring speech or music signals by channels that have a limited transmission capacity.
A first example of such a channel is a radio channel between a mobile station and a fixed base station. The available transmission capacity of this channel is limited because this channel is used by a great many users.
A second example is a recording channel which utilizes a magnetic, optical or a different recording medium such as, for example, semiconductor storage. Examples of systems utilizing such recording channels are dictating systems and machines utilizing a voice-supported user interface. In such systems it is usually desirable to provide maximum reduction of the necessary storage capacity. Prior-art systems utilize linear prediction or sub-band coding for this purpose.
In the transmission system known from said journal article, no more than a single period from a total number of periods of the quasi-periodic signal is encoded in the encoder. The transmitting means transmit the encoded signal thus obtained to the receiver through the channel. The decoder in the receiver decodes the encoded signal into a reconstructed signal. This is effected by determining the untransmitted periods of the quasi-periodic signal by means of interpolation with the periods of the quasi-periodic signal that have indeed been transmitted. It is noted that the quasi-periodic signal may be a voiced part of a speech signal. Alternatively, however, the quasi-periodic signal may be a residual signal that has been derived from a voiced part of a speech signal with a linear-prediction-based technique.
For determining the signal periods to be transmitted, it is necessary in the transmission system known from said journal article that the quasi-periodic signal be sampled at a rate that is considerably higher than is necessary according to the sampling theorem. For obtaining a reasonable quality of the reconstructed signal, it is necessary to utilize a complex algorithm for the selection of the quasi-periodic signal periods to be transmitted. Said properties of the prior-art transmission system have led to a considerable complexity of the prior-art transmission system.